Insidious / in-ˈsi-dē-əs/ adj. 

Definition: Insidious has 3 similar but different definitions, each depending on the context. Definitionally, the word can be confusing if it is used outside of these contexts.

If it is an insidious plan, something or someone is in trouble. The plan is deviously beguiling and probably designed for an insidious effect.

You can call something insidious because it hides or tries to trick you with deceit. Notice the contextual differences surrounding when you use insidious.

Probably the most common definition aligns with something that seems harmless but is really causing problems in the shadows.

Etymology: Insidious is entirely of Latin origin. The core word comes from the Latin “In” (on) and “sedere” (sit). Combined, the spelling evolved into “insedere” (lie in wait for). Later versions “insidiae” and “insidiosus” meant trick and deceptive conversely. The current form first appeared in the 16th century.

In a Sentence

Despite almost certain loss of life, the ring leader’s insidious planning seems like it should work.

The refs never catch the infractions, and your opponent is insidious in the execution of strategy.

The Covid-19 pandemic is responsible for millions of deaths worldwide, most notably because it has an insidious nature as a disease.

Synonyms 

Corrupting, Cunning

Antonyms 

Straightforward, Sincere